1. Field of the Invention
This invention relates to detector and survey systems utilising polarisable ground penetrating radiation. More particularly this invention relates to apparatus and a method for detecting both metal and non-metal objects or structures, such as pipelines which are buried in the ground wherein the apparatus can be operated in an automatic way so as to enable maps to be produced of the buried objects detected.
2. Discussion of the Background
Inductive techniques are available for locating buried metallic structures. However, such techniques cannot be used for detecting nonmetallic objects such as plastic pipes. Techniques based on RADAR have been used for geophysical surveys in which pulsed electromagnetic radiation is transmitted from a transmitting antenna located close to the ground. Any change in the dielectric properties of the sub-strata produces a reflection of the transmitted pulse which is received by a receiving antenna located above ground. The received waveform contains an echo signal at a time related to the depth of the change in strata. This technique can be employed for locating buried structures, the structure producing the echo signal. However, the echo can be very small on account of the high attenuation of the ground. Significantly larger signals result from signals off the ground and additionally direct breakthrough of energy can occur between the antennas. Thus the derived signal can be almost completely masked by unwanted "clutter signals".
It is not possible unambiguously to identify the clutter waveform separately from the target echo, for example by moving the antenna to an area of ground containing no target to provide a `reference` clutter waveform since the clutter signals are unique to any one ground position.
We have observed that the extraction of desired target reflections from the clutter can be made easier by utilising some form of difference between the signals received from the target and the clutter.
We have further observed that buried objects which are geometrically asymetrical e.g a pipe, will reflect with a different intensity those signals incident upon them which are polarized parallel to the longer axis than signals polarized perpendicular to that axis. Ground reflections however, will not depend upon polarisation in the same systematic way.
These two features can be combined to enable the orientation of a long thin target object to be found, as well as some suppression of the unwanted clutter waveform. The orientation may be discovered if the polarisation of the incident radiation can be rotated in a known way, while the clutter may be reduced by passing the received signals through a filter designed according to the characteristics of the signal being sought and the statistical properties of the unwanted clutter.
When the transmitting and receiving antennas are electrically orthogonal (which does not necessarily imply geometrically orthogonal) the radiated wave from the transmitting antenna has no effect on the receiving one, even if the two are very close or have a common centre.
Electrically orthogonal antennas have the advantage that any planar, uniform dielectric discontinuity which is perpendicular to the direction of the incident radiation produces a reflected signal which gives rise to no output from the receiving antenna. Thus, such a system is less sensitive to the presence of the ground surface than would be a pair of parallel antennas to transmit and receive the radiation. In practice there are two problems. Firstly, it is not possible to obtain perfect electrical orthogonality so that there is always some component in the output signals arising from direct breakthrough. Secondly, the ground surface is not a planar uniform dielectric discontinuity so that there is always some component due to the presence of the ground surface. When the antenna assembly is close to the ground surface the effect is superimposed on the breakthrough, and if the operational frequency is such that the ground surface is in the near field of the antenna system then the combined effect is to modify the breakthrough signal in an unpredictable way. In what follows, the term `orthogonal` will be taken to mean `as near electrically orthogonal as is conveniently practical`. A further advantage of an orthogonal antenna pair is that target objects which are long and thin, or which preferentially scatter radiation polarised in one direction, result in an output from the receiving antenna which varies with the orientation of the object relative to that of the antenna system. When the antennas are designed to transmit and receive linearly, or near linearly polarised radiation then the signal is a minimum (zero if there are no interfering signals) when the target's principal axis is parallel or perpendicular to the direction of the transmitted polarisation, and is a maximum when that axis is at 45.degree. to either of these directions. When circular polarisation is transmitted there is no amplitude variation with relative orientation, but the phase of the received signal varies with orientation. In general, elliptically polarised radiation involves both an amplitude and a phase variation and is the preferred form of radiation used by the invention.
One way of effecting the rotation of the polarisation of the energy transmitted into the ground and incident upon any buried object is by mechanical rotation of an assembly consisting of a pair of orthogonal cocentered antennas. The symmetry of the geometry dictates that the received signal can consist of up to three components: one which is independent of orientation of the target, one which is singly periodic in antenna rotation and one which is doubly periodic. The first arises from breakthrough because of inherent non-orthogonality in the design. The second is due to any non-coincidence of the mechanical centre of rotation and the electrical centre of either of the antennas, while the third arises from any scatterer, in the near of far field, which gives preferential scattering of one polarisation.
Although mechanical rotation of the antenna unit has the disadvantage of an increased complication in the hardware design because of the need to build a servo system to control the rotation over the fixed point as well as the need to cope with the flexing of the cables feeding the measurement and control electronics, especially if these include microwave connections, there are advantages. With mechanical rotation only one pair of antennas is involved and therefore there is a genuinely angle-independent contribution to the breakthrough component of the received signal, amenable to removal by subtraction. Electronic rotation has an obvious advantage because the antenna unit is mechanically stable and the polarisation direction can be changed much more quickly than a mechanical rotation allows. However, there are also apparent disadvantages to electronic rotation, but ones which can be overcome in a way which does not detract from the advantages.
For mechanically rotated antenna systems, the basic data required for polarisation processing is a set of search recordings taken as the antenna assembly is rotated above the ground location under investigation. The most obvious way of achieving this is to mount the antenna suspended from a suitable turntable and to record the received waveform at rotation intervals of 10.degree., say. When the only variable component of the received signal is doubly periodic in rotation angle, one pair of recordings made at any known angular separation (other than an integer multiple of 90.degree.) is adequate to describe the complete variation. (A convenient separation is 45.degree. or 60.degree.). In practice, there is always an angle-invariant component, usually small, which can be removed by including an extra search recording taken at 90.degree. to the first and subtracting these two. This is because a 90.degree. rotation produces a change of sign in any angle-dependent contributions which are doubly-periodic. If there is a singly-periodic component because the electrical centre of either antenna is not coincident with the mechanical centre of rotation, then a further two search recordings, taken at 135.degree. and 180.degree. say, are required to identify it.
Thus, the final search data i.e., the time waveform which exists at an arbitrary angle X relative to some reference direction on the ground is given by EQU F(t)=A(t)+B(t) sin X+C(t) cos X+D(t) sin 2X+E(t) cos 2X,
where A,B,C,D, and E are functions derived from the five search recordings as follows. Let the five recorded time waveforms be P(t), Q(t), R(t), S(t) and T(t) taken at 0, 45, 90, 135 and 180 degrees, respectively.
Then EQU A=[P+T-.sqroot.2(Q+S)+2R]/[2(2-.sqroot.2)] EQU B=-[P+T-2(Q+S)+2R]/(2-2.sqroot.2) EQU C=(P-T)/2 EQU D=[Q-S-(P-T)/.sqroot.2]/2 EQU E=(P+T-2R-.sqroot.2(P+T-Q-S)]/[2(2-.sqroot.2)].
We observe further that the resolution with which a pair of targets can be distinguished is increased for increased bandwidth of transmission, and therefore a preferred antenna design is one which allows operation over as large a frequency range as possible. Such antennas are known to be specified in their shape by angles only, and so parallel-sided dipoles are best avoided. However, the principles of the invention are most easily described with dipoles because of their diagramatic simplicity.
In UK Patent Specification No. 1532710 there is described an apparatus for locating buried objects including an antenna assembly connected to a pulse generator and to a receiver which converts echo signals into data representative of the buried object. The antenna assembly comprises a pair of orthogonally co-located two-terminal antennas. In operation, pulsed radiation is transmitted into the ground from one of the antennas and any echo signals indicative of a target, are received on the other orthogonally located antenna. That apparatus proposes that the probe should have a layer of absorber on its underside to provide distributed resistive loading. Clearly, such material is lossy when in the form of dielectric material. As an alternative the material is proposed to provide magnetic loss.
Clearly such absorber material is not a lossless dielectric. Furthermore, the thickness of the material is apparently relatively arbitrary and is not related in any way to the wavelength in the dielectric of the lowest frequency of the radiation to be transmitted.
An antenna assembly for use in the detection of objects buried in the ground which in use is in a position to transmit microwave electromagnetic radiation into the ground and to receive such radiation after reflection from said objects, according to the invention, includes a plurality of antennas each having at least one pair of elements. The elements have first surfaces, which have a cladding of substantially lossless dielectric material having a relative permittivity of at least 3.5. The cladding on the first surfaces includes a slab of dielectric material having a thickness at least one twentieth of the wavelength, .lambda..sub.d, in the dielectric material of the lowest frequency of the radiation to be transmitted from the assembly. The slab lies between the antenna elements and the ground in use of the assembly and is effective at least to increase the electrical size of the antenna elements compared with their electrical size in the absence of such slab.
Preferably the antenna will be of substantially planar construction and will have parellel upper and lower surfaces.
The coating thickness should be at least .lambda..sub.d /20 thick (where .lambda..sub.d is defined above). Preferably, the thickness should be not more than about .lambda..sub.d /5. However, where the antenna is designed to transmit (and receive) a range of frequencies, the thickness should preferably not exceed .lambda./3 where .lambda. is the wavelength of the highest frequency of the range.